Waterhemp Management in Soybeans

According to our recent SURVEY, waterhemp has become the most concerning weed in row crop production in Wisconsin. Adding to that challenge, glyphosate-resistant waterhemp populations have been confirmed in 25 Wisconsin counties (see map below). Furthermore, PPO-resistant waterhemp has been confirmed in 4 counties (Iowa, Monroe, Saint Croix, and Pierce). Resistance to ALS-inhibiting herbicides is widespread across the Midwest but has not been fully investigated in Wisconsin.

Figure 1. Distribution of glyphosate-resistant waterhemp in Wisconsin (reports from University of Illinois Plant Clinic). For additional information, see the paper from UW-Agronomy Professor Dr. Dave Stoltenberg, presented during the 2018 Wisconsin Agribusiness Classic: “Current State of Herbicide Resistance in Wisconsin”

At our on-farm research site in Chippewa county, WI, waterhemp emergence has been occurring for approximately two weeks. At our site in Grant county, WI, we noticed the first waterhemp seedlings today (06/04/2018). These plants will start growing rapidly as we move into the warmer portion of the growing season. It is important to also scout for waterhemp seedlings in fields where PRE-emergence herbicides were sprayed at planting, as the soil residual activity of these herbicides fade away.

Figure 2. Waterhemp seedling emerging in Chippewa county, WI (picture taken on 05/23/2018).

Glyphosate (Group 9), ALS-inhibiting herbicides (Group 2; e.g.: Pursuit, Classic, FirstRate), and PPO-inhibiting herbicides (Group 14; e.g., Cobra, Flextar, Cadet, Ultra Blazer) are the three primary herbicide sites-of-action (SOA) used for broadleaf weed control POST-emergence in Roundup Ready soybeans. Growers who suspect they have a glyphosate-resistant waterhemp problem should use a tank-mixture of glyphosate and a herbicide from Group 14 and/or Group 2 for effective POST-emergence control. To have success using Group 14 or Group 2 herbicides, waterhemp height should be less than 4 inches at application. Soybean fields with emerged waterhemp where glyphosate has struggled in the past should get top priority for spraying.

Group 14 herbicides are likely the best choice for POST-emergence control of glyphosate-resistant waterhemp in Roundup Ready soybeans. Growers must be aware that they may need to adjust their application practices to maximize the effectiveness of these herbicides. Group 14 herbicides are contact, cell membrane disrupters that require good spray coverage for best control. It is important to read the product label to know the required carrier volume and adjuvant selection to enhance the efficacy of the selected product.

Liberty (Group 10) in Liberty Link soybeans or Dicamba (Group 4) in Xtendsoybeans are effective options for POST-emergence control of glyphosate-resistant waterhemp, assuming one of these traits is adopted. With Liberty (contact herbicide), weeds must be treated when they are small (<4 inches) and spray coverage is important for control like for Group 14 herbicides. With dicamba, off-target movement via particle and/or vapor drift and tank-contamination are current concerns.

Because of waterhemp’s late and extended emergence window, the use of a soil residual herbicide from Group 15 (e.g., Dual II Magnum, Outlook, Warrant, Zidua) tank-mixed with the POST-emergence program is encouraged to control later flushes of waterhemp. See herbicide label for rates, application window, and restrictions.

Several effective herbicide options are available for waterhemp control in corn. Thus, we urge growers on a corn-soybean rotation to proactively manage waterhemp in corn years, including late escapes that may not impact corn yield but will replenish the soil seed-bank for subsequent years. This will increase the odds of successful control of waterhemp in subsequent soybean years. Remember, “no seed no weed”!